As the group has grown stronger in recent months, more foreigners have gone to Syria to fight in its ranks, which has forced European and US authorities to dedicate more resources to keep them from leaving their countries of origin, the sources said.
IS has become more attractive than Al Qaeda for potential fighters because it rules the territory with strict Islamic law. "IS is able to hold itself up as the true jihad," a senior US official said.
Another factor that seems to attract new fighters to the group is the its brutality, The New York Times reported.
This month IS published a video of beheading US journalist James Foley.
The Washington Post reported on Friday that at least three other hostages were subjected to waterboarding, an interrogation technique simulating the experience of drowning.
It is estimated that more than 100 US citizens have fought with rebel groups. Since January the number of US citizens fighting for the rebels in Syria has doubled, a source said.